1. Field of the Invention
The invention relates generally to communication systems and, more particularly, to time division multiplexed communication systems. This invention has applicability to communication systems operating with bandwidth-limited broadcast spectrums.
2. Description of the Related Art
A typical communication system 10 using time division multiplex techniques is shown in FIG. 1A. An antenna 101 is coupled to a two position switch 102. When the switch 102 is coupled to a first terminal, the signal received by the antenna 101 is applied to low noise amplifier 103. The output signal from the low noise amplifier 103 is applied to mixer unit 104. The mixer unit 104 also receives a signal from the local oscillator unit 105. The signals from the low noise amplifier 103 and the local oscillator unit 105 are combined in mixer unit 104 and applied to filter unit 106. The filtered signal from filter unit 106 is applied to an IF (intermediate frequency) unit 107. The output signal from the IF unit 107 is applied to demodulation unit 108. The demodulation unit 108 recoveries the data (information) that has been encoded in the transmitted signal and applies the data to an output terminal. When the switch unit 102 is in the second position, the output signal from power amplifier is applied to the antenna for transmission of information. The power amplifier 109 receives signals from mixer unit 110. The mixer unit 110 combines signals from the local oscillator unit and from a unit (not shown) that has encoded an intermediate signal with data to be transmitted. Typically, the intermediate frequency is fixed. However, the local oscillator unit 105 has a controllable voltage and can be implemented, for example, by a voltage controlled oscillator. The communication system 10 is multiplexed in that the system is adapted to alternatively receive and transmit signals.
Referring to FIG. 1B, the operation of the duplex mode of the time division multiplexed communication system 10 is illustrated. For purposes of the operation of the time division multiplexed system, time is divided into a series of equal intervals. During alternate intervals, the system is transmitting, i.e., switch 102 has coupled the antenna 101 to the power amplifier 109. During the alternate intervals when the system 10 is not transmitting, the switch 102 couples the antenna 101 to the low noise amplifier 103. During these intervals, the system is in a receiving mode. Thus, the system alternately transmits data and receives data.
The broadcast frequency spectrum is divided into a plurality of a multiplicity of broadcast bands, each band being reserved for a particular purpose. Referring to FIG. 2, a broadcast band ranging from 902 MHz to 928 MHz is used as an example. The broadcast band is divided into channels, 10 channels are shown by way of illustration in FIG. 2. As will be seen from FIG. 2, the transmitted signal will be spread and in fact can overlap the signals of a neighboring channel. Because of this overlap, the signals associated with each channel are offset for purposes of illustration and clarity. The signals in each channel have a central frequency that is a combination of the local oscillator frequency (LO) and the intermediate frequency (IF). However, a noise signal having a frequency of the local oscillator LO will also be broadcast. The noise signal for channel 3, i.e., LO3, is shown in FIG. 2. As can be seen from this illustration, this noise signal can be positioned in the broadcast band and, therefore, can interfere with other information signals being broadcast in the band.
A need has been felt for apparatus and an associated technique that features the reduction of noise signals in the broadcast band resulting from the transmission of a signal having the frequency of the local oscillator. It is another feature of the present technique that it be applicable to all the channels of broadcast band.
The aforementioned and other features are provided, according to the present invention, by selecting the local oscillator frequency such that the difference between the local oscillator frequency causes a signal with the local oscillator frequency to fall outside of the broadcast band while the carrier signal having a combination of the local oscillator frequency and the intermediate frequency be the designated frequency of the associated channel. In addition to the selection of frequency of the local oscillator, the low-side injection and high-side injection, i.e., the selection of the upper or lower sideband, can be used to determine whether the carrier signal frequency is below or above the local oscillator frequency. By selecting both the local oscillator frequency and the sideband used as the carrier signal, the noise signal generated by the communication within the broadcast band can be reduced.